1. Field of the Invention
The invention is generally related to network elements for communication between computers, and more specifically related to the creation and elimination of entries in a forwarding memory.
2. Description of Related Art
Communication between computers has become an important aspect of everyday life in both private and business environments. Computers converse with each other based upon a physical medium for transmitting the messages back and forth, and upon a set of rules implemented by electronic hardware attached to and programs running on the computers. These rules, often called protocols, define the orderly transmission and receipt of messages in a network of connected computers.
A local area network (LAN) is the most basic and simplest network that allows communication between a source computer and destination computer. The LAN can be envisioned as a cloud to which computers (also called endstations or end-nodes) that wish to communicate with one another are attached. At least one network element will connect with all of the endstations in the LAN. An example of a simple network element is the repeater which is a physical layer relay that forwards bits. The repeater may have a number of ports, each endstation being attached to one port. The repeater receives bits that may form a packet of data that contains a message from a source endstation, and blindly forwards the packet bit-by-bit. The bits are then received by all other endstations in the LAN, including the destination.
A single LAN, however, may be insufficient to meet the requirements of an organization that has many endstations, because of the limited number of physical connections available to and the limited message handling capability of a single repeater. Thus, because of these physical limitations, the repeater-based approach can support only a limited number of endstations over a limited geographical area.
The capability of computer networks, however, has been extended by connecting different subnetworks to form larger networks that contain thousands of endstations communicating with each other. These LANs can in turn be connected to each other to create even larger enterprise networks, including wide area network (WAN) links.
To facilitate communication between subnets in a larger network, more complex electronic hardware and software have been proposed and are currently used in conventional networks. Also, new sets of rules for reliable and orderly communication among those endstations have been defined by various standards based on the principle that the endstations interconnected by suitable network elements define a network hierarchy, where endstations within the same subnetwork have a common classification. A network is thus said to have a topology which defines the features and hierarchical position of nodes and endstations within the network.
The interconnection of endstations through packet switched networks has traditionally followed a peer-to-peer layered architectural abstraction. In such a model, a given layer in a source computer communicates with the same layer of a peer endstation (usually the destination) across the network. By attaching a header to the data unit received from a higher layer, a layer provides services to enable the operation of the layer above it. A received packet will typically have several headers that were added to the original payload by the different layers operating at the source.
There are several layer partitioning schemes in the prior art, such as the Arpanet and the Open Systems Interconnect (OSI) models. The seven layer OSI model used here to describe the invention is a convenient model for mapping the functionality and detailed implementations of other models. Aspects of the Arpanet, however, (now redefined by the Internet Engineering Task Force, or IETF) will also be used in specific implementations of the invention to be discussed below.
The relevant layers for background purposes here are Layer 1 (physical), Layer 2 (data link), and Layer 3 (network), and to a limited extent Layer 4 (transport). A brief summary of the functions associated with these layers follows.
The physical layer transmits unstructured bits of information across a communication link. The repeater is an example of a network element that operates in this layer. The physical layer concerns itself with such issues as the size and shape of connectors, conversion of bits to electrical signals, and bit-level synchronization.
Layer 2 provides for transmission of frames of data and error detection. More importantly, the data link layer as referred to in this invention is typically designed to "bridge," or carry a packet of information across a single hop, i.e., a hop being the journey taken by a packet in going from one node to another. By spending only minimal time processing a received packet before sending the packet to its next destination, the data link layer can forward a packet much faster than the layers above it, which are discussed next. The data link layer provides addressing that may be used to identify a source and a destination between any computers interconnected at or below the data link layer. Examples of Layer 2 bridging protocols include those defined in IEEE 802 such as CSMA/CD, token bus, and token ring (including Fiber Distributed Data Interface, or FDDI).
Similar to Layer 2, Layer 3 also includes the ability to provide addresses of computers that communicate with each other. The network layer, however, also works with topological information about the network hierarchy. The network layer may also be configured to "route" a packet from the source to a destination using the shortest path. Finally, the network layer can control congestion by simply dropping selected packets, which the source might recognize as a request to reduce the packet rate.
Finally, Layer 4, the transport layer, provides an application program such as an electronic mail program with a "port address" which the application can use to interface with Layer 3. A key difference between the transport layer and the lower layers is that a program on the source computer carries a conversation with a similar program on the destination computer, whereas in the lower layers, the protocols are between each computer and its immediate neighbors in the network, where the ultimate source and destination endstations may be separated by a number of intermediate nodes. Examples of Layer 4 and Layer 3 protocols include the Internet suite of protocols such as TCP (Transmission Control Protocol) and IP (Internet Protocol).
Endstations are the source and ultimate destination of a packet, whereas a node refers to an intermediate point between the endstations. A node will typically include a network element which has the capability to receive and forward messages on a packet-by-packet basis.
Generally speaking, the larger and more complex networks typically rely on nodes that have higher layer (Layers 3 and 4) functionalities. A very large network consisting of several smaller subnetworks must typically use a Layer 3 network element known as a router which has knowledge of the topology of the subnetworks.
A router can form and store a topological map of the network around it based upon exchanging information with its neighbors. If a LAN is designed with Layer 3 addressing capability, then routers can be used to forward packets between LANs by taking advantage of the hierarchical routing information available from the endstations. Once a table of endstation addresses and routes has been compiled by the router, packets received by the router can be forwarded after comparing the packet's Layer 3 destination address to an existing and matching entry in the memory.
As packets are being transmitted and received between endstations, they must traverse intermediate nodes in which a network element receives a packet and forwards it towards its final destination. When a packet arrives at such an element, the packet's headers (or portions of the headers) are collected and an attempt is then made to match using hardwired logic the destination of the packet to a known Layer 2 destination address in an address table stored in a forwarding memory. Alternatively, a matching cycle may be performed in software based on the Layer 3 information contained in the received packet's header.
As a high performance device, the forwarding memory must be able to produce the matching entry from among thousands of entries in a relatively short time. This speed is important because the decision to forward the received packet to its destination must be made as soon as possible after receiving the packet to make room for other arriving packets.
Existing techniques for implementing the forwarding memory include the use of a content addressable memory (CAM) to store a Layer 2 address table. The CAM is typically implemented as a hardwired logic integrated circuit and is nicely suited for the job of storing an address table in a network element. The high performance offered by a CAM, however, comes at a high cost because of its unique and complex hardware requirements. Therefore, a CAM for use as a forwarding memory in networking applications is typically selected to provide space for only a limited number of entries.
One way to make efficient use of the forwarding memory is to reduce the number of entries by discarding stale entries after a given time has elapsed, the rationale being that since there has not been much activity in the past, there will not be much in the future. The price paid for using this approach is that if a subsequent matching packet does arrive fairly quickly after the entry is discarded, then a new identical entry must be created. This will interfere with both header processing and forwarding memory operations. It would be desirable, therefore, to devise a technique for even more efficient use of the forwarding memory in a networking application. What is needed is a network element that makes efficient use of the forwarding memory in order to forward packets faster and save costly storage space in the forwarding memory.